Scientific Background

A ShakeMap (Wald et al., 1999) is a representation of ground shaking produced by an earthquake. Each earthquake produces a range of ground shaking levels at sites throughout the region depending on distance from the hypocenter, the rock and soil conditions at sites, and variations in the propagation of seismic waves from the earthquake due to complexities in the structure of the Earth's crust.

Part of the strategy for generating rapid-response ground motion maps is to determine the best format for reliable presentation of the maps given the diverse audience, which includes scientists, businesses, emergency response agencies, media, and the general public. In an effort to simplify and maximize the flow of information to the public, Wald et al. (1999a, 1999b, 1999c) developed a means of generating not only peak ground acceleration and velocity maps, but also an instrumentally-derived, estimated Modified Mercalli Intensity map. This map makes it easier to relate the recorded ground motions to the expected felt and damage distribution. The Instrumental Intensity map is based on a combined regression of recorded peak acceleration and velocity amplitudes.

Data gaps are common, particularly for very small events and events near or outside the edge of the network. In order to stabilize contouring and minimize the misrepresentation of the ground motion pattern due to data gaps, the data are enriched with predicted values in areas without data. Given the epicenter and magnitude, peak motion amplitudes in spare regions are estimated from the available Ground Motion Predictive Equations (Akkar and Bommer, 2007, for strong motion and Malagnini et al., 2002, for weak motion) together with those by the National Seismic Hazard Working Group (2004) for the generation of the national map of seismic hazard.

Further developments about the ShakeMap model were recently published by Worden et al. (2010); at the same time new relationships between the ground motion parameters and the instrumental intensities are available (Worden et al., 2012).

Limitations of Current Maps

The ShakeMap approach is simple and approximate. It is not considered the direction of rupture nor the peak motions are corrected by a directivity term. The empirical predictive approach only gives average peak ground motions values so it does not account for all the expected variability in motions, other than the aforementioned site amplification variations. Actual ground motions show significant variability for a given distance, magnitude, and site condition and, hence, the scenario ground motions are more uniform than would be expected for an actual earthquake. The true variations are partially attributable to 2D and 3D wave propagation, path effects (such as basin edge amplification and focusing), differences in motions among earthquakes of the same magnitude, and complex site effects are not accounted for by our method.

Note: ShakeMaps are generated automatically following moderate and large earthquakes. These are preliminary ground shaking maps, normally posted within several minutes of the earthquake origin time. The acceleration and velocity values are raw and are at least initially, NOT checked by humans. Further, since ground motions and intensities typically can vary significantly over small distances, these maps are only APPROXIMATE. At small scales, they should be considered unreliable. Finally, the input data is raw and unchecked, and may contain errors. (See Disclaimer)